The snapshot is written synchronously to disk. When the JS heap is large,
it may introduce a noticeable "hitch".

Previously, node-heapdump first forked the process before writing the snapshot,
making it effectively asynchronous. However, it broke the comparison view in
Chrome DevTools and is fundamentally incompatible with node.js v0.12. If you
really want the old behavior and know what you are doing, you can enable it
again by setting NODE_HEAPDUMP_OPTIONS=fork in the environment:

$ env NODE_HEAPDUMP_OPTIONS=fork node script.js

On UNIX platforms, you can force a snapshot by sending the node.js process
a SIGUSR2 signal:

$ kill -USR2 <pid>

The SIGUSR2 signal handler is enabled by default but you can disable it
by setting NODE_HEAPDUMP_OPTIONS=nosignal in the environment:

On UNIX systems, the rule of thumb for creating a heap snapshot is that it
requires memory twice the size of the heap at the time of the snapshot.
If you end up with empty or truncated snapshot files, check the output of
dmesg; you may have had a run-in with the system's OOM killer or a resource
limit enforcing policy, like ulimit -u (max user processes) or ulimit -v
(max virtual memory size).